In deference to those who’d like to remain spoiler free, let’s just say Helena Bonham Carter certainly leaves her mark as the diabolically dark witch Bellatrix Lestrange in Harry Potter and the Deathly Hallows — Part 1. But the actress, who already terrorized young audiences this year as the Red Queen in Alice in Wonderland, wasn’t originally supposed to play Bellatrix. Helen McCrory (The Queen) originally had the part for 2007’s Harry Potter and the Order of the Phoenix, but had to bow out after she got pregnant. “So they came to me,” says Bonham Carter. “And I loved it. I love magic, I love witches, I love the whole [Harry Potter] world. I was all too happy to play a witch.” (McCrory was eventually cast as Bellatrix’s sister Narcissa Malfoy for Harry Potter and the Half-Blood Prince and the two Deathly Hallows films.)

So how did Bonham Carter bring one of Harry Potter’s biggest enemies on screen? When she first showed up on set for the fifth Harry Potter film, “There wasn’t a huge amount on the page originally,” says the actress, 44. “I think I probably made her a bit more insane and unhinged then she was meant to be. I wanted to be conspicuous. So the [rotten] teeth was my idea, because she had been in prison so long. I wanted her to be quite savage. And I wanted that corset. It was sort of an Amazon thing. Bellatrix means a warrior. I wanted her to be sexy and revolting at the same time. At one point she might have been attractive, but no longer.” Most important of all: Bellatrix’s dense thatch of hair. “Here’s the thing: If you have messy hair, you don’t have to worry all the time about making it all perfect. It was a lot to do with keeping [all the hair and makeup people] away.”

Of course, playing one of Voldemort’s most eager minions wasn’t just about the look; you also had to get the magic right. “We went to wand school for about three weeks,” Bonham Carter says of herself and her fellow Death Eaters. “We took it very very seriously. I actually got a wand blister on the [middle] finger. They kind of had different names for all the different moves, Latin ones. It was sort of based on fencing. You also always have to have an intention behind the spell. You just feel totally emasculated on set when you’re waving a bit a wood around and nothing’s coming from it. It’s very difficult to remember that you’ve actually got the intention to kill behind it. It’s not all fun and laughs; it’s actually quite hard work.” So when Bonham Carter got to cast a spell as Bellatrix actually setting Hagrid’s hut ablaze in The Half-Blood Prince, the chance to get to see the fruits of her magical labor with her own eyes was, according to Bonham Carter, quite like a rather popular iPhone game about a pack of furious fowls. “I actually pointed my wand and it blew up! The power! The power was just like Angry Birds, but big [as] life.” Naturally.

For more on the Harry Potter Villains, including Voldemort, Severus Snape, and Lucius Malfoy, pick up this week’s issue of Entertainment Weekly.